In internal combustion engines, the piston ring shows a barrel profile on its contact face against a cylinder wall of the combustion engine. The barrel profile may be symmetrical or more commonly asymmetrical. If asymmetrical, the highest point of the profile exhibiting a maximum curvature is slightly displaced to the ring lower flank.
In operation of the combustion engine, the ring profile changes due to wear, assuming a slightly conical shape. The worn profile has strong influence on oil scraping and the contact pressure distribution, and consequently on the ring wear rate.
Due to the engine operating conditions, the temperature on the ring inside diameter is greater than on the outside diameter. Such thermal gradient causes a different expansion on the ring, being greater on the outside diameter than on the inside diameter. As a result of this different expansion, the butt end regions apply much more pressure against the cylinder wall than the other ring region, causing a greater wear on these butt end regions, even an early failure due to overpressure.
As a consequence of the greater wear rate, the hard layer usually applied on the ring contact face may be completely worn out at the butt end regions, causing a shortening of the edge live of the piston ring (References [1] to [3]).
In order to solve such localized wear problems, several solutions based on two main concepts are known:    1) A different free shape of the piston ring, so as to compensate in part for the deformations which occur during its operation and which cause the higher wear at the butt ends.
Patent document EP 0 545 094 A1 discloses a self-tensioning piston ring showing an uneven radial pressure distribution in such a way that the pressure maximum is located to the regions on the right and the left side of the butt ends and that the radial pressure at the ring joint drops down to zero.
Similar solutions are disclosed in patent documents JP 2001-263488 A and DE 43 10 249 C1.    2) A thinner cross-section at the area next to the butt ends, so that even with the deformations that arise during operation, a better accommodation of the butt end areas to the cylinder can occur. The cross-section is reduced in such areas next to the butt ends, no matter whether radially or crosswise, making those regions more flexible. These more flexible regions transmit less pressure to the cylinder wall, thus minimizing wear on the piston ring itself.
Patent document EP 0 253 069 A1 discloses a self-tensioning piston ring for high-speed machines, which is provided with a sharp edge at the underside of each butt end region. Each sharp edge turns into a bevel which extends over a certain length of the perimeter of the piston ring.
Patent document US 2002/0041071 A1 discloses a compression piston ring which includes a ring joint whose gap is defined by first and second end faces. A ring back is situated diametrically opposite to the ring joint. In comparison to the ring back, the region of the first and second end face is reduced exclusively.
A similar solution is disclosed in patent document JP 2000-120866 A.
In addition, it is also known that increasing the width of the ring contact face which effectively contacts the cylinder wall reduces the contact pressure and consequently the wear rate as well (References [4] and [5]).